AVRCP: Pick a media player ASAP

Fix things to pick a "default media player" as soon as we can so that
connected devices have something to send media commands to.  This is
also in line with the spec.

Previously we were not detecting when we didn't have a media player
selected and selecting one.

Test: boot with com.android.music and connect to carkit, check logs
Bug: 36016671
Change-Id: I89175a3d513635d7cf8b75f5010967b99d5490aa